An example of this would be some salamanders which retain the gills which, in most amphibians, are lost upon reaching adulthood. It's assumed that at some point in the past, their gills were lost just like all others, but some genetic change caused them to be retained, at a point where it was evolutionarily advantageous or neutral.

There are several kinds of pedomorphism which may appear independently or in combination:

Neoteny, in which somatic (or physical) development is slowed, resulting in a sexually mature juvenile or larval form.